The present invention relates to a visible light emitting device comprising an LED or laser diode and a phosphor. More particularly, the present invention relates to a white light emitting device comprising a near UV/blue LED chip or laser diode and one or more near UV/blue excitable phosphors.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. They are particularly useful as display lights, warning lights and indicating lights or in other applications where colored light is desired. The color of light produce by an LED is dependent on the type of semiconducting material used in its manufacture.
Colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been produced from Group III-V alloys such as gallium nitride (GaN). To form the LEDs, layers of the alloys are typically deposited epitaxially on a substrate, such as silicon carbide or sapphire, and may be doped with a variety of n and p type dopants to improve properties, such as light emission efficiency. With reference to the GaN-based LEDs, light is generally emitted in the UV and/or blue range of the electromagnetic spectrum. Until quite recently, LEDs have not been suitable for lighting uses where a bright white light is needed, due to the inherent color of the light produced by the LED.
LEDs rely on its semiconductor to emit light. The light is emitted as a result of electronic excitation of the semiconductor material. As radiation (energy) strikes atoms of the semiconductor material, an electron of an atom is excited and jumps to an excited (higher) energy state. The higher and lower energy states in semiconductor light emitters are characterized as the conduction band and the valence band, respectively. The electron, as it returns to its ground energy state, emits a photon. The photon corresponds to an energy difference between the excited state and ground energy state, and results in an emission of radiation.
Recently, techniques have been developed for converting the light emitted from LEDs to useful light for illumination purposes. In one technique, the LED is coated or covered with a phosphor layer. By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum may be generated. Often, a combination of LED generated light and phosphor generated light may be used to produce the visible light (e.g. white). The most popular white LEDs consist of blue emitting GaInN chips. The blue emitting chips are coated with a phosphor that converts some of the blue radiation to a complimentary color, e.g. a yellow-green emission. Together, the blue and yellow-green radiation produces a white light. There are also white LEDs that utilize a UV emitting chip and phosphors designed to convert the UV radiation to visible light. Typically, two or more phosphors are required to produce the white light. Known phosphors for use with LEDs in producing white light include cerium doped yttrium aluminum garnet Y3Al5O2:Ce3+ (“YAG:Ce”) as well as Tb3Al4.9O12:Ce (“TAG:Ce”).
There are different “whites”, “warm whites” and “cold whites” being the most common description. Scientifically, all chromaticies corresponding to black body spectra making up the Planckian locus are “whites”. A color near to this locus, a “white” is characterized by a deviation from the coordinates lying on the Planckian locus on the CIE chromaticity diagram and is perceived as a slight coloration called a tint. In standards like the automobile industries SAE J578, the maximum tint permitting the designation “white” is specified.
Although known LED-phosphor combinations can be effective, the nature of such devices limits them to emitting only one shade of “white”. That is, each specific phosphor and LED combination will produce a different “white” emission. Each phosphor will have a certain specific emission spectrum. Likewise, a certain LED semiconductor chip will have a unique emission spectrum. These emissions can be plotted as x and y coordinates on the CIE chromaticity diagram. Only colors on the line connecting the two points can be produced by the LED/phosphor combination. Thus, such a combination will be limited to producing only a single white. As standardized in SAE J578, however, the automobile industry defines the “white” emission region as encompassing a range of “whites” having slightly different emission spectra. Known phosphor/LED combinations cannot produce all “whites” within this region. Thus, it would be advantageous to produce a phosphor coated LED having a “white” color not producible using known phosphor and LED combinations.